WordPress uses TinyMCE, so there's an argument that these questions are relevant. However, the same can be said about JQuery. I don't see us being the place to go for JQuery questions.
Should we answer this question here? Or migrate it over to SO?
Both TinyMCE and jQuery are external libraries maintained by external teams. TinyMCE also offers some proprietary extensions that may or may not work with WordPress.
In the case of that specific question, I'm very tempted to migrate it.
In the case of "how do I add this TinyMCE plugin" questions ... those are somewhat specific to WordPress due to the way WordPress bundles its libraries together and implements them on the front-end. So it's really a case-by-case decision.
If you ever feel that a question is off-topic, though ... flag it. One of the moderators will take a look and either migrate the question, ask a clarifying question like Rarst did, or clear the flag.
Such are usually off topic... problem is it might not be off topic, but lacking details that makes it specific to WordPress. I usually ask how is this specific to WP and if person can't write that out then close/migrate.